1. Field of the Invention
The present invention relates to fiber optic coils. More particularly, this invention pertains to a sensor coil with reduced bias and to a tool for use in forming such a coil.
2. Description of the Prior Art
One application of a fiber optic sensor coil is in a fiber optic gyroscope. The fiber optic gyroscope generally comprises the following components: (1) a light source, (2) a beamsplitter (either a fiber optic directional coupler or an integrated-optics Y-junction), (3) a fiber optic coil, (4) a polarizer (and sometimes one or more depolarizers), and (5) a detector. Light from the light source is split by the beamsplitter into copropagating and counterpropagating waves travelling in the sensing coil. The associated electronics measures the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias or error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. Such factors are both time-varying and unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
One approach to attain a reduction of sensitivities arising from environmental factors has involved the use of various symmetric coil winding configurations. In such coils, the windings are arranged so that the geometrical center of the coil is located at the innermost layer while the two ends of the coil are located at the outermost layers.
N. Frigo has proposed the use of particular winding patterns to compensate for non-reciprocities in xe2x80x9ccompensation of Linear Sources of Non-Reciprocity in Sagnac Interferometersxe2x80x9d. Fiber Optics and Laser Sensors I, Proc. SPIE Vol. 412 p. 268 (1989). Furthermore, U.S. Pat. No. 4,793,708 of Bednarz entitled xe2x80x9cFiber Optic Sensing Coilxe2x80x9d teaches a symmetric fiber optic sensing coil formed by dualpole or quadrupole winding. The coils described in that patent exhibit enhanced performance over the conventional helix-type winding.
U.S. Pat. No. 4,856,900 of Ivancevic entitled xe2x80x9cQuadrupole-Wound Fiber Optic Sensing Coil and Method of Manufacture Thereofxe2x80x9d teaches an improved quadrupole-wound coil in which fiber pinching and microbends due to the presence of pop-up fiber segments adjacent the end flanges are overcome by replacing such pop-up segments with concentrically-wound walls of turns for climbing between connecting layers. Both of the aforementioned United States patents are the property of the assignee herein.
While the above-identified symmetrical winding techniques and geometries help minimize bias error, they, as well as all sensor coils, are based upon coil winds in which overlying coaxial layers are wound from flange to flange (or end to end) about a mandrel. Such general geometry, for example, creates a plurality of fiber turnaround regions that join the end turns of wound layers. As layers, sometimes adjacent to one another (in the case of a quadrupole-wound coil, the fiber must climb two layers in a turnaround region), are wound in opposite senses, the optical fiber must necessarily cross over turns of an underlying layer when the direction of winding reverses, creating small-radius microbends that are known to create undesired phase shifts (in addition to other error sources introduced by the coil winding process).
The present invention provides, in a first aspect, a sensor coil for sensing a physical parameter, such as in a fiber optic gyroscope. Such sensor coil includes an optical fiber. The fiber comprises a plurality of coaxial layers of turns. The layers of turns are encapsulated with potting material to form a fiber pack.
A climbing turn joins a layer of turns to an overlying layer of turns. Each of the climbing turns is located outside the fiber pack.
In a second aspect, the invention provides a tool for forming a fiber optic sensor coil. Such tool includes a reel for receiving and accommodating an optical fiber during winding. The reel comprises a cylindrical mandrel and a pair of inner flanges. Each of the inner flanges is substantially planar and fixed to one of the opposed ends of the cylindrical mandrel so that the inner flanges are substantially parallel to one another.
A pair of outer flanges is provided. Each of the outer flanges is substantially planar and each is in contact with the outwardly-facing surface of one of the inner flanges of the reel. Each of the flanges of the reel has a pair of apertures defining a wedge therebetween.
In a third aspect, the invention provides a method for forming a potted sensor coil in which an optical fiber is formed into a plurality of coaxial cylindrical layers of turns on a takeup reel that includes a cylindrical mandrel with generally-planar flanges fixed substantially parallel to one another at opposed ends of the mandrel.
The method is begun by providing a radially-directed wedge in each of the flanges of the takeup reel. Thereafter, the optical fiber is threaded the therearound as the coil is wound so that the coil is formed with climbing layers located outside a potted fiber pack.
The foregoing and additional features of the invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures in which numerals, corresponding to numerals of the written text, point to the various features of the invention with like numerals referring to like features throughout both the drawing figures and the written description.